Volume 2, Issue 2, Summer 1985

Building Better Bat Houses

Bat houses have been used for more than 60 years. In Europe, thousands have been placed in national forests, and hundreds more are now used in the United States. Providing artificial roosts is increasingly important as bats lose their traditional roosts in old trees and buildings. Some bat houses are occupied within days, but others are ignored even when bats are excluded from roosts in nearby buildings. Recent research on bat preferences indicates how bat houses can be made more attractive.

Mark E. Hodgkins studied 17 Oregon (U.S.) nursery roosts in man-made structures to determine preferences of the Yuma Myotis (Myotis yumanensis), the Big Brown Bat (Eptesicus fuscus) and the Pallid Bat (Antrozouspallidus). All three species roosted within 300 meters of a river, lake or pond. However, they differed significantly in their choice of roost size and shape and in their tolerance of temperature extremes. Nursery colonies roosted in exceptionally warm places. Colonies of the Yuma Myotis roosted where temperatures ranged up to 98 F, while Big Brown and Pallid Bats chose roosts that did not exceed 90 F.

Many roosts, especially small ones, cool at night, failing well below the
optimum temperature range for nursery colonies. Probably for this reason, Hodgkins found that roost orientation to the sun was important. Roosts facing east to southeast have optimal exposure to the morning sun, begin heating sooner and have higher daily heat gain. He also found that most nursery colonies lived in relatively large structures, capable of providing the most stable high temperatures, even though the bats often occupied only a small part of the available area.

Based on his field observations, Hodgkins recommended two basic bat house designs, one for little brown bats, such as the Yuma Myotis (Fig. 1, a), and another for the Big Brown, Pallid and other larger bat species (Fig. 1, b). Currently available generic designs are mostly small, often do not provide temperature stability within the desired range and may not meet the crevice width preference of a particular species. These seem to be used mainly when better alternatives are unavailable.

The Missouri Department of Conservation designed one of the more successful bat houses (plans available from Fish and Wildlife Research Center, 1110 College Avenue, Columbia, MO 65201), but it is too large (7.5 x 3.9 ft.) and costly for most people to build. Of the small generic types, the design illustrated in Figure 2 appears to be one of the best. Hodgkins' new plans (Fig. 1) seem to offer an excellent compromise, providing a variety of crevice space options and increased temperature stability at a reasonable size and cost.

Bat nursery colonies usually prefer the most stable temperatures available, in the 80-90 F range, though free-tailed and myotis bats often tolerate temperatures in excess of 100 F. Since appropriate temperature is important to bat house success, several geographic factors must be considered. With increasing latitude and altitude, lower temperatures require that bat houses be oriented to receive maximum solar radiation (especially in the morning). They also may require tar paper or dark colored shingles on the top and upper parts of the sides to help absorb and hold heat. One person in a cool climate failed to attract bats until he added a thermostat-controlled heating unit.

The extent of solar radiation needed depends on the local climate. In exceptionally hot climates bats may even require shaded roosts. Testing the temperature in a given bat house design and location by placing a maximum/minimum thermometer inside for several days and making appropriate adjustments, will greatly increase the probability of success. Larger structures provide greater stability, and tight-fitting tops and sides reduce heat loss. Attachment to a building may further increase stability. Taller roosts provide temperature gradients, permitting bats to move vertically to find their preference. Do not paint or varnish bat houses, or use lumber that has been chemically treated with wood preservatives.

Bat houses located near a permanent source of water, especially a lake or river, are by far the most likely to attract bats. They should be roughly 12-15 feet above ground, sheltered as much as possible from wind. Ideal roosting sites are rare even in nature, so don't be discouraged if conditions in your bat house are not perfect. If you live in good bat habitat you may be surprised at how easily bats can be attracted. Regardless of your bat house design or its ability to attract bats, it will make a great conversation piece!

Fig. 1. Bat house plans for small (a) and large (b) crevice-roosting bats. The frame is covered with l x 8" shiplap lumber (c), preferably redwood or cedar. Except for the bottom, left open for bat entry, the structure should be as airtight as possible. Lengths of vertical dividers for the small bat design (a) are 4-6". Total estimated materials costs are $56 (a) and $87 (b). Detailed plans and instructions are available for $1 from BCI. illustrations courtesy Mark E. Hodgkins.